I'm playing build 2016-05-19.4781a80 but I have seen the behaviour in older builds. The AI landed an attack ship on an unguarded outpost of mine. It leaves an attack ship on the outpost which sits there until another ship arrives to replace it. The troop ship arrives & the attack ship leaves. The troop ship gets slaughtered by my attack fleet because it failed to leave the combat ship there the 1 extra turn it needed.

This is actually golden for stealing the ai's kill. Sometimes I'll just wait for an ai player to knock out another ai's defences then move in & the last moment.